1. Field of the Invention
The present invention relates to an earthquake resisting design method of a prestressed concrete construction (hereinafter, referred to as “PC construction”). The PC construction of the present invention is defined to indicate a configuration in which high-strength precast prestressed concrete (PCaPC) members (column, beam) are joined by PC binding juncture with a prestressing tendon.
2. Description of the Related Art
A reinforced concrete construction (RC construction) of the related art, being inexpensive, highly rigid and superior in occupant comfort, is used in buildings such as collective housing and offices in many cases.
In contrast, a prestressed concrete construction (PC construction) is configured to resist an envisioned load by applying a prestress to a cross section of a concrete member in advance so as to be applied to buildings having a large span beam or beams and columns that support a heavy load. Since having high restorability in comparison with RC construction, it can maintain a required goodness after earthquakes.
A plurality of technologies (patents) are known about the PC construction. A first known technology is a connecting structure between a column and a beam, which have a junction between a precast concrete column and a precast concrete beam. It is characterized in that a junction having a cross section protruding from a side surface of the beam and a bottom surface of the beam is provided at an end of the beam, coupling reinforcement rods configured to couple the beam and the column are disposed on a lower portion of the beam and an upper portion of the beam of the junction, and a prestressing tendon is arrayed at a position nearer to a neutral shaft of a cross section of the beam than these coupling reinforcement rods, thereby coupling the beam and the column (JP07-042727B).
In this connecting structure between the column and the beam, the coupling reinforcement rods are disposed on the upper and lower portions of a height of the beam at the coupling portion, and the prestressing tendon for introducing the prestress is arranged at positions near the neutral axis of the cross section, so that the upper and lower reinforcement rods at the connection bear a large deformation caused by a load at the time of earthquakes to absorb large deformation energy. It is said that the prestressing tendon, which performs mainly a binding functions at the junction between the column and the beam, is subjected to a smaller deformation than the reinforcement rods, damage at the time of earthquake is small, and safeness is achieved.
A second known technology is a PC binding juncture construction between a precast concrete beam and column which is a construction in which an unbonded prestressing tendon is utilized for introducing a prestress, and a precast concrete beam is joined to a precast concrete column by binding juncture. It is characterized in that a resilient member, configured to prevent crush of concrete at an end of the beam by absorbing a compress deformation, is installed on a side surface of the column at a portion subjected to compression by a rotational deformation caused by lifting of the beam (JP2002-004417A).
It is said that the PC binding juncture construction between a precast concrete beam and column contributes to architecture of an RC system building in which a rigid frame body is not damaged even by the large earthquake which is considered to occur once in a hundred years, or damage can be repaired by replacing an impact material.
A third known technology is a self-seismic isolation construction method for an RC system construction in which an unbonded prestressing tendon is utilized for introducing a prestress, and a precast concrete beam is joined to a precast concrete column by binding juncture. The self-seismic isolation construction method for the RC system construction is characterized in that the unbonded prestressing tendon is penetrated through the precast concrete beam in a longitudinal direction, both end portions of the unbonded prestressing tendon are fixed to the precast concrete column, so as to achieve a configuration which allows lifting of a column-beam juncture interface in association with a resilient expansion deformation of the unbonded prestressing tendon in accordance with a horizontal force of an earthquake or the like (JP2002-004418A).
It is said that according to the self-seismic isolation construction method for the RC system construction, an own natural period of the RC system construction may be elongated without using a seismic isolation apparatus and a vibration control apparatus. It is also said that since the seismic isolation apparatus and the vibration control apparatus, and also maintenance in association therewith are not necessary, it contributes considerably to cost reduction, and is superior in occupant comfort.
Furthermore, a fourth known technology is an earthquake resisting structure built by a PC binding construction method. The earthquake resisting structure by the PC binding construction method is characterized in that a body frame, having a beam and columns at both ends thereof as a minimum unit, has junctions between the beam and the columns configured as rotatable junctions, which bears mainly a perpendicular load, and is constructed by binding juncture in which a prestress is introduced into an unbonded type prestressing tendon penetrated through the beam in the axial direction and also through the columns. A horizontal resistance member, configured to absorb energy by yielding before the body frame becomes damaged at the time of earthquake, is affixed to a side surface portion of the body frame, and it is a plate member having a length to straddle the rotatable junctions at the both ends of the beam. Both side positions of the rotatable junctions are coupled thereto by the binding juncture in which the prestress is introduced into the prestressing tendon (JP2005-171643A).
Since this earthquake resisting structure by the PC binding construction method has a configuration in which the columns and the beam of the body frame are joined by binding juncture in which the prestress is introduced into the long unbonded type prestressing tendon, which mainly bears a perpendicular load, distortion of the prestressing tendon is averaged over the entire length. Therefore, distortion of the prestressing tendon falls within the range of the resilient limit even at the time of great deformation, and structural safety is high. It is said that the body frame can easily follow the great deformation at the time of earthquake, and after the earthquake has gone, restoration is performed as an effective advantage of the prestress introduced into the prestressing tendon, so that a residual deformation is restored to zero.
It is said that in the first known technology, the reinforcement rods disposed in upper and lower portions of the beam bear a large deformation caused by a load at the time of an earthquake, and absorb large deformation energy; the prestressing tendon, which introduces the prestress and which is arranged at a position near a neutral axis of the junction between the column and the beam, is subjected to a smaller deformation than the reinforcement rods, damage at the time of the earthquake is small, and safeness is achieved. However, this has a problem that energy is absorbed by a plastic deformation of the reinforcement rods in the same manner as the RC design of the related art, and hence the residual deformation of the reinforcement rods after the earthquake is large, and cannot be repaired.
The second known technology provides a configuration in which the resilient member, configured to prevent crush of concrete at the end of the beam, is provided on the side surface of the column at a portion subjected to compression by the rotational deformation caused by lifting of the beam. However, it is apparent that providing a plurality of notched depressions for mounting the resilient members on a plurality of side surfaces of the column at the same level makes loss of cross section of the column itself, and thereby reduces significantly the strength thereof. And this has a problem that because of lack of a member configured to support the end portion of the beam, repeated earthquake forces cause downward slippage at the junction to the column, which may easily cause fracture of the unbonded prestressing tendon itself and breakage of a binding junction between the beam and the column, and thereby produces a very high risk of destruction of the construction.
It is said that in the third known technology, the unbonded prestressing tendon is penetrated through the precast concrete beam in the longitudinal direction, the both end portions thereof are fixed to the precast concrete columns, so as to allow lifting of the column-beam juncture interface in association with the resilient expansion deformation of the unbonded prestressing tendon in association with the horizontal force of the earthquake or the like. However, this has a problem that the fixation of the unbonded prestressing tendon in this case is 80% of a standard yield load of the prestressing tendon from the description saying “not specifically new, and is implemented by a method described in a PC standard of Architectural Societies,” in the same manner of the second known technology, repeated earthquake forces cause downward slippage at the junction to the column because there is no member configured to support the end portion of the beam, and thereby there is a very high risk of destruction of the construction caused by the fracture of the prestressing tendon.
In the fourth known technology, the horizontal resistance member, configured to absorb energy by yielding before the body frame becomes damaged at the time of the earthquake, is affixed to the side surface portion of the body frame, and it is the plate member having a length to straddle the rotatable junctions at the both ends of the beam, and the both side positions of the rotatable junctions are coupled thereto by the binding juncture in which the prestress is introduced into the prestressing tendon. It is said that consequently, damage is encouraged to be concentrated on the horizontal resistance member, to cause plastic deformation therein, absorption of earthquake energy, reduction of response, and thereby realization of debilitating effect. However, it is still the plastic design as conventional, and hence the plastically deformed horizontal resistance member cannot be repaired. Therefore, all the horizontal resistance members need to be replaced after the earthquake, which requires troublesome task in field work, so that there is a problem of significant increase in cost.
As a common problem in the second to the fourth known technologies, grease used as a filler of the unbonded prestressing tendon is subjected to an oil separation phenomenon with time, which significantly impairs corrosion control performance. Therefore, it is not preferable to use the unbonded prestressing tendon in the PC binding juncture structure between the column and the beam.
A current earthquake resisting design standard in Japan allows damage of a structure with a seismic intensity 5 upper or so, and allows even a collapse as long as design ensures safety of human life. There is a report saying that many damages occurred at the time of a huge earthquake exceeding a seismic intensity 6, such that buildings of the RC structure, the S structure, and the SRC structure were collapsed or significantly deformed (plastic deformation of an inter-story deflection angle of 1/100 or larger) and broken, and that the residual deformation remains after the earthquake and cannot be repaired.
The term “seismic intensity” is an index indicating degree of shaking of the earthquake at a certain point, and indicates earthquake scales used by Japan Meteorological Agency (Japan meteorological Agency seismic intensity scale).
In particular, Japan is a country having often earthquakes, and has a soil which can be subjected to a great earthquake disaster any time. A current design method for constructing buildings of RC structure or S structure is “plastic design,” wherein the reinforcement rods and steel frames are designed to be used as far as in plastic region at the time of earthquake. On such a soil, this is not a design method suitable for this state of country. In addition, buildings designed on the basis of a theory of absorbing energy by a plastic deformation, which is a basic of the reinforcement rods construction, absorb energy of earthquake by a plastic deformation of a panel zone. Consequently, there is a problem that the panel zone is subjected to a shear failure, and damage and residual deformation caused by the earthquake are severe and hence restoration is impossible after the earthquake has gone. In a word, in the RC rigid frame structure of the design method of the related art, the destruction at the time of the large earthquake occurs certainly in the panel zone (column-beam junction), and hence the entire construction is destructed in a manner that the column is destructed in first by the shear failure in the panel zone.
In any cases, in the PC construction of the related art, a tension introducing force of the prestressing tendon arranged on a cross section of a member is designed to be 80% of the standard yield load (Py) of the prestressing tendon at the time of completion of the fixation. The current earthquake resisting design method against the earthquake allows yield of the prestressing tendon under the maximum design load in the same manner as the RC construction. Consequently, since there is no sufficient available capacity preserved in the prestressing tendon under permanent load, the prestressing tendon yields and is plastically deformed at the maximum design value, so that a loss of superior restorability of the PC construction is accompanied. Then, the force of restoring the deformation of the construction members vanished and, after the earthquake has gone, residual deformation remains. Therefore, generated cracks cannot be closed, and the cracks progress larger with passing of time. This affects a construction frame negatively, whereby a lifetime of usage is significantly reduced.
Since the “plastic design,” wherein deformation of the panel zone is allowed for absorbing earthquake energy, is used in the same manner as the RC construction or the like, the shear failure in the panel zone (column-beam junction) cannot be avoided at the time of a large earthquake. In addition, at the time of the huge earthquake with a seismic intensity 6 or greater, which exceeds an earthquake resisting design level, since a cogging for supporting the beam is not provided at the binding junction between the column and the beam, the beam slips downward and the prestressing tendon is subjected to precedence fracture, and a shear failure of the beam occurs together with the breakage of the construction members, so that there is a risk of collapse of the building. There is also a problem that a surface area of a loop of a load deformation curve is smaller than that of the RC construction, energy consumption by the plastic deformation of the construction is smaller in the historical characteristics, and hence it is said not to be a desirable nature against the large earthquake.